If there are multiple LSPs (label switched paths) that all originate on one LSR (label switched router) and all terminate on another LSR, then these LSPs can be merged (under control of the head-end LSR) into a single FA-LSP (forwarding adjacency-label switched path) using the concept of link bundling which is described in detail in draft-kompella-mpls-bundle (see for example www.ietf.org/internet-drafts/draft-kompella-mpls-bundle-05. txt).
For example, to improve scalability of MPLS-TE (multiple protocol label switching protocol-traffic engineering) it may be useful to aggregate TE LSPs. The aggregation is accomplished by:
an LSR creating a TE LSP;
the LSR forming a forwarding adjacency out of that LSP (advertising this LSP as a link into an internal routing protocol such as ISIS or OSPF);
allowing other LSRs to use forwarding adjacencies for their path computation; and
nesting of LSPs originated by other LSRs into that LSP by using a label stack construct.
The details of this approach and the label stack constructs can be found in draft-ietf-mpls-lsp-hierarchy,—(see for example www.ietf.org/internet-drafts/draft-ietf-mpls-lsp-hierarchy-02. txt).
This approach will be described further by way of example with reference to FIG. 1. Shown is a network of hierarchically connected nodes including at a lowest level in a hierarchy four nodes 1, 2, 10, 11, at a higher level in the hierarchy four nodes 3, 4, 8, 9, and at a highest level in the hierarchy three nodes 5, 6, 7. Also shown are four end user terminal devices T1, T2, T3 and T4 connected to nodes 1, 10, 2, 11 respectively. For a terminal T1 connected to node 1 to communicate with a terminal T2 connected to node 10, use may be made of a first forwarding adjacency established between node 5 and 7, a second between nodes 3 and 8, and a third between nodes 1 and 10. The result is that a user packet to be forwarded from T1 to T2, say an IP (Internet Protocol) packet, will have the user packet header, a first label understood by nodes 1 and 10 representing the LSP between these nodes, a second label understood by nodes 3 and 8 representing the forwarding adjacency between these nodes, and a third label understood by nodes 5 and 7 representing the forwarding adjacency between those nodes. Thus, for each packet there is the original user header, an IP header in this example, plus three labels. For packets which are long, the overhead introduced by these three additional labels may not be significant. However, for short packets, the overhead can be a significant percentage of the overall packet size. In some networks, for example networks in which there is significant voice traffic, there is a high percentage of the overall packet flow which has short packet length. An example packet size distribution is shown in FIG. 2 obtained during a seven minute interval over a real network in March of 1998 (see www.caida.org/outreach/resources/learn/packetsizes) where it can be seen that a significant fraction of the packets have a length less than 100 bytes. It is noted that in today's applications the number of small packets would be even larger than that shown in FIG. 2 because the number the voice-over-IP and IP teleconference applications has increased. Using the above described hierarchical labeling approach in a network with this type of packet length distribution would result in a significant increase in overall overhead.